Interpenetrating polymer networks, IPN's, are combinations of two (or more) polymers which have been polymerized and/or crosslinked in the immediate presence of each other [L. H. Sperling, J. Polym. Sci., Macromolecular Reviews, 12, 141 (1977)]. These materials have been shown to make excellent noise and vibration damping materials, U.S. Pat. No. 3,833,404; toughened plastics, German Offenlegungsschrift No. 2,153,987; improved soft contact lenses, German Offenlegungsschrift No. 2,518,904; and many other materials.
Like the related materials, polymer blends, grafts, and blocks, most of the IPN's exhibit greater or lesser degrees of phase separation. The outstanding properties of IPN's arise from the synergistic combination of two dissimilar materials, i.e., an elastomer and a plastic. The IPN's can be considered as a subclass of the graft copolymers, where the crosslinks outnumber the graft sites, and/or dominate the physical properties of interest.
A major shortcoming of these IPN's is their thermoset nature. After one or both polymers form a network, shapability is reduced, and processability in terms of melt flow is very poor.
Simple mechanical blends of two polymers have long been known, and many offer significant improvements over their respective homopolymers. Generally comprised of two (or more) linear polymers, they are thermoplastic, rather than thermoset.
The IPN's noted above depend on covalent crosslinks; however, three types of non-covalent crosslinked IPN's have been generally recognized (as indicated in certain U.S. patents noted below). These make use of multiblock copolymers (U.S. Pat. No. 3,265,765), semicrystalline polymers, and polymers bearing ionic charges (U.S. Pat. Nos. 3,264,272 and U.S. Pat. No. 3,404,134). When two such polymers are blended mechanically, especially such that the development of dual phase continuity is encouraged, a class of materials designated as mechanically blended thermoplastic IPN's arises (U.S. Pat. Nos. 4,041,103; 4,101,605; 3,862,106).
Notwithstanding these prior art teachings, there remains a need for thermoplastic IPN compositions of improved processing and mechanical behavior. It is therefore the general object of the present invention to provide such improved thermoplastic IPN compositions and processes for making such compositions.